Winding temp

ABSTRACT

882,940. Thermal switches. TEXAS INSTRUMENTS Inc. April 13, 1960 [April 15, 1959], No. 13275/60. Class 38 (5). A thermal switch comprises an insulating cup-shaped casing 1 carrying fixed contacts 3, 5 co-operable with movable contacts 21, 23 mounted on a bimetal plate 15 loosely carried by an adjustable post 11 and having ears 16 co-operable with projections 27 to prevent plate rotation. A heater 35 comprises a loop 47 engaging an insulating cover 29, and legs 39 having lugs 41 sprung into grooves 45 in the casing 1, one heater terminal 43 being connected to contact terminal 7 which, with its fellow terminal 9 and heater terminal 44 is sealed to the casing 1 by a potting compound 57. The loop 47 of the heater may be replaced by a straight ribbon-like portion, by a rectangular portion having an aperture adjacent the post 11 or by a multi-looped wire, with wire legs and terminals, fitted in a groove in the cover 29, Figs. 6, 7, 8 and 9 (not shown), all the heaters operating at a non-incandescent temperature. The switch may be used to protect the windings of a split-phase electric motor against overload and stalled-rotor conditions, Figs. 10 and 11 (not shown), the switch and heater design being such that the plate 15 and the motor windings heat and cool at approximately the same rate to prevent the windings exceeding a critical temperature during the initial period of the overload, Fig. 13 (not shown).

H. D. EPSTEIN THERMOSTATIC SWITCH Sept. 10, 1963 3 Sheets-Shee t 1 Original Filed April 15, 1959 Se t. 10, 1963 H. D. EPSTEIN 3,103,566

THERMOSTATIC SWITCH Original Filed April 15, 1959 3 Sheets-Sheet 2 Sept. 10, 1963 H. D. EPSTElN THERMOSTA'IIC SWITCH Original Filed April 15, 1959 5 Sheets-Sheet 5 F'IGIO.

FIGQ.

C O 5 7 W E 5 T B G m .M a P E L WT 0 Tc i G W H m D mw WC S P M M m @5 W T W G mm c D r1 5 m D W MJJ I B 1 w 5 M w m 2. mnzzuk TIME w W? 3 P. 1 M R E W T P. m m N W c T C s 6 C D mm D W L Am H m 1 W m P M PM E M T Er C T 5 G D mm m w m H 9 I QEE TiME United States Patent 3,103,566 THERMOSTATIC SWITCH Henry David Epstein, Cambridge, Mass, assign'or to Texas Instruments Incorporated, Dallas, Tern, a corporation of Delaware Continuation of application Ser. No. 306,686, Apr. 15,

1959. This application Feb. 1, 1962, er. No. 172,374 19 Claims. (Cl. 200-122) This invention relates to thermostatic switches, and with regard to certain more specific features, toan 1mproved heater type of thermostatic switch operable as an overload protector for motors and the like.

This application is a continuation of my copendlng application Serial No. 806,686, filed April 15, 1959, now abandoned, for Thermostatic Switch.

Among the several objects of the invention may be noted the provision of a thermostatic switch more particularly for motor protection circuits (but applicable to others), having an improved form of heater of comparatively high thermal mass adapted to prolong the off time of such a protection circuit upon cycling of the switch thermostat, thereby to reduce the total heat which willbe put into the protected motor and thereby minimize its temperature rise; the provision of a switch of the class described which employs a nonincandescent heater adapted also to minimize flash-over and thereby simplify problems of electrical clearance in the switch design; the provision of a thermostatic switch having such a nonincandescent heater permitting its construction as a low-cost improved structural element adapted conveniently to be snapped into mounted position without the use of complicated fasteners; and the provision of a switch of the class described which is reliable, compact in form and economical. Other objects and features will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of parts which will be exemplified in the structures hereinafter described, and the scope of which will be indicated in the following claims.

In the accompanying drawings, in which several of various possible embodiments of the invention are illustrated,

FIG. 1 is an enlarged plan view of a switch embodying one form of the invention;

FIG. 2 is a front elevation of FIG. 1;

FIG. 3 is a bottom plan view of FIG. 1;

FIG. 4 is a View similar to FIG. 1, showing the switch with its cover removed;

FIG. 5' is a vertical section taken on line 5-5 of FIG. 1;

FIGS. 6, 7 and 8 are perspective views illustrating alternative forms of heater elements;

FIG. 9 is an inside plan view on a smaller scale of a modified form of cover adapted to receive the heater element shown in FIG. 8;

FIGS. 10 and 11 are circuit diagrams illustrating various applications of the invention;

FIG. 12 is a graph illustrating diagrammatically certain functions of prior devices; and

' FIG. 13 is a similar graph illustrating certain functions of devices made according to the invention.

ice

are not entirely satisfactory because of the tendency of I motor temperatures to increase by escalation under rapid recycling conditions caused by the high rate of cooling of the thermostat'when deprived of the motor heat by insufficient heat transfer. Under the latter conditions auxiliary heaters have been employed near the thermostat in the switch so as to prolong the off time of the switch under cycling conditions. It is to the form and characteristics of such a heater that the present invention relates.

It will be understood that formerly thermostatic switches for motor protection have been constructed with built-in heater elements for their current-heated thermostatic elements, intended to minimize the rate of recycling and consequent motor temperature increase in response to faults, and more particularly heavy faults due to locked rotor conditions or the like. Such former heater elements have usually operated at incandescence and per se had a relatively low thermal mass. However, their casings supplied some desired thermal mass. Operation at incandescence had various disadvantages, among which was the ionization of air around the heater with a greater tendency to flash-over between switch parts in the ionized air. As a consequence, greater clearances were required between such parts than if they had been located in nonionized air. Also, the incandescent heaters had deleterious effects upon switch housings usually made of a synthetic resin. The former incandescent heaters also complicated the problem of making suitable electrical connections thereto. They were also dificult to assemble.

According to the present invention, an improved simple form of heater which is convenient to make and to assemble is employed in a protective thermostatic switch, the heater being constructed to operate below incandescence and to have a comparatively high thermal mass per se (in addition to the usual thermal mass supplied by the housing), whereby there is increased the total thermal mass available for decreasing the cooling rate of the heater. The result under cycling conditions is that a winding or the like to be protected has a somewhat greater time between cycles within which to cool, so as to minimize temperature climbing or escalation of motor temperature.

Referring now more particularly to FIGS. 1-6, there is shown at numeral 1 an enclosing cup-shaped casing base part composed of suitable molded material which is both an electrical and heat insulator. In the base are fixed contacts 3 and 5 from which extend spot-welded terminals 7 and 9, respectively. A suitable material is one of the thermosetting synthetic resins or the like. Terminals 7 and 9 are held aligned by suitable lugforming portions 2 on the bottom of the base 1.

Threaded into the base is a supporting post 11, initially adjustable in position by means of a screw-driver slot 13. At its upper end, the post supports a flexible preferably bimetallic thermostatic electrically conductive disc or plate 15, composed of face-bonded component layers 17 and 19. The component 17 is composed of material of relatively low coefficient of thermal expansion, whereas the component 19 is composed of material of a relatively high coefiicient of thermal expansion. The disc 15 has movable contacts 21 and 23 bonded to its lower face, in the region of opposite ears 16.

The connection between the disc 15 and post 11 is afforded by a fairly loose engagement between an end 12 of the post and an opening 14 in the disc 15. A confining washer 18 is held in position above the disc 15 by means of a headed portion 20 of the end 12. As normal current flows through the disc 15 it maintains a normal temperature, as for example up to 75 C., and maintains the cold bowed position such as shown, wherein the movable contacts 21 and 23 engage the fixed contacts 3 and 5, respectively. In response to excessive current, the disc 15 will be current heated, and at a temperature of, for example, 135 C. will snap from the bowed position shown in FIG. to a conversely bowed position shown in FIGS. and 11, wherein the movable contacts 21 and 23 separate from the fixed contacts 3 and 5, respectively. As appears in FIG. 4, the base 1 is of more or less rectangular external shape, rounded at the corners, being provided with a suitable internal pocket 25 closely surrounding the disc 15. This pocket includes projections 27, which prevent rotation of the disc loosely mounted on the post 11.

At numeral 29 is shown a corner-rounded hollow rectangular cover composed of a material such as a glass fibre filled polyester resin. Such material insulates against heat loss and insulates electrically. it is also capable of effectively holding a substantial amount of heat. Such a material is what may be referred to as heat-conservative material. The cover 29 has a top portion 31 which is substantially flat on the inside, and a rectangular skirt portion 33 adapted to telescope over the base 1. The skirt 33 is internally grooved as shown at 34.

At numeral 35 is shown an extensiveresistance heater strip (Nichrome, for example) formed with a flat serpentine portion 37 adapted closely to overlie the disc and having springy side leg portions 39 formed to engage the ends of base 1 and, when assembled, to lie in the grooves 34 of the skirt 33. At their lower ends the legs 39 are crimped inward to form lugs '41and are bent outward to form terminals 43 and 44. The lugs 41 are adapted to spring into. a recessive groove 45 formed around the outside bottom of the base 1. Thus are formed snap-latching means 41, 45 between the springy side legs 39 of heater 35 and sides of the casing 1. Terminal 43 is soldered or welded to terminal 7. The serpenti'ne fiat top 37 of the heater 35 comprises a loop 47 having two side arms 49 extendinghorizontally to the legs 39. Cross sections of the heater strip 35 are diimensioned so that heating in response to overload current will raise its temperature, but not to incandescence nor to a temperature which would cause thermal degradation of the insulating materials used in the cover 29 and base part 1, or which would cause these insulating materials to give ofi gases and char or otherwise cause deterioration. V

The flat inside of the top 3-1 of cover 29 has flatwise contact with the flat top of the heater strip 35, and the grooves 34 in the ends of skirt 33 have fiatwise contact with the heater legs 39. The insides of legs 39 also have flatwise engagement with the outside ends of the base 1, being thus enclosed. Notches 55 in the cover 29 are provided for exit of the heater terminals 9, 43, 44. By

means of this arrangement the switch as a whole provides a comparatively high heat mass so that most or the heat 4 only slowly dissipated. Note in this connection that th base 1 and cover 29 form a closely fitting housing around the disc 15 and heater 35. Therefore, any temperature once attained by the heater 35 and the disc 15 in response to current reduces slowly after the current is shut off. The close physical contact between the cover 29 and heater 35 contributes substantially to this cheat since the heater transmits heat rapidly to the cover, to be stored therein, and the latter is composed of heat-conservative material which loses heat only slowly.

It will also be seen that assembly of the heater 35 is readily accomplished by snapping it into position over the cup-shaped base 1 with the latch-forming lugs 41 in catch-fiorming groove 45 and its flat portion bridging the open end of the base. Then the cover 29 may be telescoped into closing position, also bridging the open end of the base and contacting the heater. Thereafter the terminals 7 and 43 may be joined and, lafiter calibration at 13, a potting compound such as shown by the stippling 57 is introduced to maintain assembly.

The form of the heater may be varied, as indicated in FIGS. 6, 7 and 8. In FIG. 6 it has a straight flat ribbonlike top 59, the side legs 61 extending \as lugs 63 and terminals 64 and 65. This figure illustrates a point to be kept in mind regarding all forms of the heater shown, namely, that before assembly the extensions of the legs from the top initially converge slightly, so that when a heater is applied to base 1, the legs will cling flatwise to sides of the base, the lugs snapping into the groove 4-5.

In the alternative form of heater shown in FIG. '7, the top is in sheet-like rectangular form, as shown at numeral 67, wherein is an opening 69 to be positioned adjacent'to and around the upper end of the post 11. This provides for placing the heater and cover therefor closer to the disc 15. The size of the opening 69 also controls the cross section presented to current flow. The legs of this formare shown at 71, its lugs at 73 and the terminals at 74 and 75.

In the form of the invention shown in FIG. 8, the heater is constituted by a preferably flat grid of wire 77 of serpentine or looped shape having converging end leg portions 79, crimps'80 and terminals 81. In order that this storm of heater may have close contact with its cover, the latter is modified on its inside as shown in FIG. 9. This cover 29A includes a serpentine groove 30 formed'in the inside surface of its top for nesting the serpentine wire 77 (FIG. 8). 'Its sides include grooves 32 for nesting the legs 79. The sides are also notched as at 34 for exit of the terminals 81. The wire 77 is of sufficient diameterto generate the heat required, without reaching incandescence, like the other heater forms described. The loops of the FIG. 8 serpentine form may be more closely related than in the case of an incandescent heater because of the reduction in danger of flash-over due to ionization of such incandescent heaters.

In FIG. 10 is shown an application of the invention to motor protection. At M is shown a motor to be protected, having a run winding 83 and a start winding85. These are connected at 87 to one side L2 of a circuit wherein is a main line switch 89. The run winding 83 and start winding 85 are connected in parallel between point 87 and a point 91 through parallel-connected lines 93' and 95. Point 91 is connected to the other side L1 of the line through the disc 15 and its contacts 3, 5, 21 and 23, when closed. The line 95 includes the heater element 35' of FIGS. 1-6, or any of those shown in FIGS. 7, 8 and 9. This heating element such as 35 is in series with the start winding 85. A conventional centrifugal starting switch 97 is located in the line 95. Start switch 97 is closed and opens automatically as running speed is approached. Under locked rotor conditions it automatically closes. The switch as a whole is assumed to be in heat-exchange relationship to the motor windings, mounted upon its case,

for example, or among its windings.

When line switch 89 is closed (switch 97 closed), both windings 83 and 85 are energized. Switch 97 delivers current to the heater 35 and series-connected disc 15. As

the motor accelerates to normal speed, centrifugal switch 97 opens, only the run winding 83 thereafter receiving current. Under normal conditions, the current drawn is insufiicient substantially to heat disc 15, heater 35 or overheat the windings 83, 85. Under overload conditions,

more current will be drawn and will tend to heat the disc 15, heater 35 and, depending on the nature of the overload, one or both of the windings 83 and 85. Some heat from the windings will inthe course of time reach the switch and flow into and through its casing to the disc 15 series with both paralleleconnected run and start windings 83and 85.

Refierriug now to FIGS. 12 and 13, the operating ad'- vantages of the invention will be described. In FIG. 12

. are shown, for example, the operating characteristics of fiormer nonheater type switches. It will be understood that the former type of switch under consideration is connected, for example, as in FIG. or in FIG. 11. Graph 99 indicates winding temperatures and graph 101 disc temperatures. Thus when employing dormer nonheater type switches when the switch .89 is closed, the mlotor starts with switch 97 closed. By the time the motor reaches running speed,.switch 97 opens. A running temperature lO f (approximately 75 C. is thenreached, as shown by line 117. Under normal continuous running conditions this temperature is approximately maintained in the motor windings and somewhat less in the heater 35 and disc 15. Disc will not trip frotn'its solidiine closed-circuit position.

Assume first (still employing :a former switch) a drastic overload condition such as may occur under locked rotor conditions. Switch 97 will be closed and the temperature of the motor windings will rise rapidly. So also will the heater and disc temperatures rapidly rise, the'l-atter primarily because the disc is subject to current-induced self-heating therein. When the temperature of the disc reaches the 135 C. value shown by line 115, it open the circuit (see: point 102). The temperature of the motor windings will somewhat overshoot 135 C., as shown at point 106. After the circuit opens, the disc 15 will cooltopoint 105 on line 117 corresponding to 75 C., but in the meantime the winding 85 cools more slowly, say to point 109. The cooled disc then again closes the circuit if the tault has not cleared, re-energizing the motor windings. Again the temperatures of the disc and windings rise. However, the rise in the ease of the winding escalates from point 109 to point 111 during the period that the disc heats to the opening temperature of 135 C.

(see point 113). As FIG. 12 shows for the old form of switch, recycling such as just described continues for several cycles, during which the winding temperature continues to escalate above critical unallowable winding temperatures indicated: by line 114. Such unallowable temperatures may be such as set by an electrical safety code.

Still referring to FIG. 12 (nonheater type), ultimately enough heat will be exchanged between the hot motor and the switch so as to increase the cycling period in other words, the heat of the motor windings finally reaches the disc in the switch to slow down the periodic switch reclosures. Thus after several cycles, dormer switches reach the stable cycling wherein the temperature swings of the motor windings and the switch become approximatelyequal, as indicated toward the right Y on FIG. 12. However, in the meantime, critical winding temperatures have been exceeded, as indicated at the lett of this'figure. It is these that the invention avoids, as illustrated in FIG. 13.

to open the circuit at the time that point 121 is reached of the slow cooling of heater 35, so that the reclosing term perature of 75 C. (line 117) is not quickly reached, as indicated at point 125. The motor windings then [have time within which to cool to 43; point such as 127. Thus upon reclosur'e or the circuit, the winding temperature escalates from the lower temperature point 127 (compare point 109 in FIG. 12) The heater according to the invention operates at a low temperature as above set torth and has a comparatively.high-heat mass which, with the highheat-Inass or the casing, that which is responsible for this optimum lower cooling rate of the disc 15 shown in FIG. 13. As a result, the maximum temperature reached by the windings on the second closure cycle in FIG. 13 will be such as shown at point 129, which is below the critical winding temperature indicated again by line 114. Ultimately, again, heat from the motor windings will reach the switch to augment the he at of heater 35 as applied to disc 15, thus increasingly delaying the of successive reolosure cycles, as indicated to the right of FIG. 13. However, unlike the case of FIG. 12, the motor windings have been prevented from reaching the critical winding temperature.

Besides the operating advantages above referred to, it

will be observed that the switch has important structural cence admits of the desired heat-conductive contact between the heater and the cover, without damage to-the latter. It also avoids ionization within the switch housing, thus minimizing flash-over, so that the switch as a whole may be built more compactly. Moreover, connect-ions such as shown at 43 or those under contacts 3 and 5 may be of the solder, rather than the welded type, when a nonincandescent heater is employed, it being understood that an incandescently operating heater might melt a soldered connection.

An advantage of the terminals 9, 44 and (7, '43) extending outside of the switch housing is that the switch may be outside wired as desired by a user.

\ In view of the above, it will be seen that the several objects of the invention are achieved and other advantag'eous results attained.

' As variouschanges could be made in the above construlctions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

- What is claimed is:

1. A thermostatic switch comprising an electrically insulating casing having a bottom and a surrounding wall having a margin spaced cfrom the bottom, fixed contacts on said bottom, a composite snap-acting thermostatic plate mountedon said bottom and located within said wall, movable contacts carried on said plate engageable.

, 7 and disengageable with said fixed contacts, an electrically conductive U-shaped heater'having springy side legs and a joining bridging portion therebetween which spans said margin adjacent said plate, said side legs including snaplatching means operative on the sides of the casing a cover having a skirt exteriorly telescoping said wall and contacting said bridging portion in heat-conductive relation thereto throughout most of its bridging extent, both legs'of said U-shape extending between the telescoping skirt and said wall and being surrounded thereby in substantial heat-conductive contact therewith throughout most of the lengths of said legs.

2. A thermostatic switch comprising an electrically insulating casing having a bottom and a surrounding wall having a margin spaced irom the bottom, fixed contacts on said bottom having electrically conductive extensions [from the casing forming two terminals, a composite snap-acting thermostatic plate mounted above said bottom with its margin movable closely to said wall margin, and having its component of relatively low coefiicient of thermal expansionon its side adjacent the wall margin, movable contacts mounted on the other side of said plate. engageable and disengageable with said fixed contacts, an electrically conductive U- shaped heater having springy legs and a joining bridging portion therebetween, said side le'gs'including snap-latching means operative on the sides of the casing, said bridging portion spanning said wall margin substantially in its plane and near the movable plate margins, a cover having a skirt exteriorly telescoping said wall and contacting said bridging portion in heat-conductive relation thereto throughout most of its bridging extent, both legs of said U-shape extending between the telescoping skirt and said wall and being surrounded thereby in substantial heat-conductive contact therewith throughout most of the lengths of said legs, electrically conductive lugs on the legs extending outward from the end of said telescoping skirt, one of said lugs being soldered to one. of said terminals and the other lug also extending from said skirt to form a third switch' terminal, whereby said heater when excited is adapted to operate substantially to deliver heat to the casing and cover at a nonincandescent temperature low enough to preserve said soldered connection.

3. A thermostatic switch according to claim 2, wherein said cover is interiorly substantially that and has an inside grooved portion nesting said bridging portion of the, heater in a plane close to said plate.

4. -A thermostatic switch comprising a base, fixed con-- tacts on said base, a composite snap-acting thermostatic plate mounted on said base, movable contacts carried on said plate engageable and disengageable with said fixed contacts, said base having cover support means; adjalcent peripheral portions of said plate, said cover support means extending from the base to a region beyond the plate, an electrically conductive heater having end portions and an adjoining bridging portion therebetween spanning opposite parts of said cover support, means, a cover having a skirt exteriorly surrounding said coversupport means and contacting said bridging portion in heat-conductive relation thereto throughout most of its bridging extent, the skirt and parts of said supports of composite snap-acting thermostatic plate mounted on said base and located within said peripheral portion, movable contacts carried on said plate engageable and disengageable with said fixed contacts, an electrically conductive heater having end portions and a joining bridging portion therebetween spanning opposite sections of said peripheral portion, a cover having a skint-exteriorly telescoping said peripheral portion and contacting said bridging portion} in heat-conductive relation thereto throughout most of its bridging extent, at least a portion of said cover beingv disposed in abutting relation with respect to said peripheral portion of saidicasing and sandwiching said end portions 0t said heater therebetween, and means securing said cover, casing and heater together as arunit.

7. A thermostatic switch comprising an electrically insulating casing having a base and a peripheral portion an electrically conductive heater having end portions and a joining bridging pontion therebetween spanning opposite sections of said peripheral portion, a cover having a skirt exteriorly telescoping said peripheral portion and contacting said bridging portion in heat-conductive relation thereto throughout most of its bridging extent, atleast a portion of said cover being disposed inabutting relation with respect to said peripheral portion of said casing and sandwiching said end portions of said heater there'- between, and means securing said cover, casing and heater together as a unit and electrically conductive lugs on theend portions of the heater extending outward from the end of said telescoping skirt, one of said lugs being electrically connected to pile of said terminals and the other lug also extending from said skirt to form a third switch terminal, whereby said heater when energized is adapted to operate substantially'to deliver heatto the casing and cover.

8. A thermostatic switch comprising an electrically insulating casing having a base and a peripheral portion surrounding said base, fixed contacts on said base having electrically conductive extensions from the casing forming two terminals, a composite snap-acting thermostatic plate mounted on said base and located within said penipheral portion, movable contacts carried on said plate engageable and disengageable with said fixed contacts, an electrically conductive heater having end portions and a joining bridging portion therebetween spanning opposite sections of said peripheral portion, a cover having a skirt exteriorly telescoping said peripheral portion and contacting said bridging portion in heat-conductive relation thereto throughout most of its bridging extent, at least a porv tion of said cover being disposed in abutting relation with respect to said peripheral portion of said casing and sandwiching said end portions of said heater therebetween, and means securing said cover, casing and heater together as a unit and electricallyconductive lugs on the end porsaid cover sandwiching said end portions of the heater therebetween, and means securing said cover, base and cover support means and the cover at a nonincandescent electrically conductive extensions from the casing formtemperature low enough not deleterionsly to afiect said resinous material. 1 0

6. A thenmostatic switch comprising anelectrically in sulatin-g casing having a base and a peripheralportion surrounding said base, fixed contacts on said base, a

tions of the heater extending outward from the end of said telescoping skirt, one of said lugs being soldered to I one of said terminals and the other lug also extending from said skirt to form a third switch terminal, said heater when energized being adapted to operate substantially to deliver heat to the casing and cover at a nonincandescent temperature low enough to preserve said soldered connection. 1

9. A thermostatic switch comprising an electrically insulating casing having a base and a peripheral portion surrounding said base, fixed contacts on said base having ing two terminals, a composite snap-acting thermostatic plate mounted on said base and located Within said peripheral portion, movable contacts carried on said plate engageable and disengageable with said fixed contacts, an

electrically conductive heater having end portions and a joining bridging portion therebetween spanning opposite sections of said peripheral portion, a cover having a skirt exteriorly telescoping said peripheral portion and contacting said bridging portion in heat-conductive relation thereto throughout most of its bridging extent, at least a portion of said cover being disposed in abutting relation with respect to said peripheral portion of said casing and sandwiehing said end portions of said heater therebetween, and means securing said cover, casing and heater together as a unit and electrically conductive lugs on the end portions of the heater extending outward from the end of said telescoping skirt, one of said lugs being electrically connected to one of said terminals and the other lug also extending from said skirt to form a third switc terminal, said heater when energized being adapted to operate substantially to deliver heat to the casing and cover at a nonincandescent temperature low enough not to deleteriously afiect said casing and cover.

10. The thermostatic switch as set forth in claim 9 and wherein said casing and cover are formed of a resinous material.

References Cited in the file of this patent UNITED STATES PATENTS 2,891,124 Vaughan June 16, 1959 

4. A THERMOSTATIC SWITCH COMPRISING A BASE, FIXED CONTACTS ON SAID BASE, A COMPOSITE SNAP-ACTING THERMOSTATIC PLATE MOUNTED ON SAID BASE, MOVABLE CONTACTS CARRIED ON SAID PLATE ENGAGEABLE AND DISENGAGEABLE WITH SAID FIXED CONTACTS, SAID BASE HAVING COVER SUPPORT MEANS ADJACENT PERIPHERAL PORTIONS OF SAID PLATE, SAID COVER SUPPORT MEANS EXTENDING FROM THE BASE TO A REGION BEYOND THE PLATE, AN ELECTRICALLY CONDUCTIVE HEATER HAVING END PORTIONS AND AN ADJOINING BRIDGING PORTION THEREBETWEEN SPANNING OPPOSITE PARTS OF SAID COVER SUPPORT MEANS, A COVER HAVING A SKIRT EXTERIORLY SURROUNDING SAID COVER SUPPORT MEANS AND CONTACTING SAID BRIDGING PORTION IN HEAT-CONDUCTIVE RELATION THERETO THROUGHOUT MOST OF ITS BRIDGING EXTENT, THE SKIRT AND PARTS OF SAID SUPPORTS OF SAID COVER SANDWICHING SAID END PORTIONS OF THE HEATER THEREBETWEEN, AND MEANS SECURING SAID COVER, BASE AND HEATER TOGETHER AS A UNIT WITH THE PLATE CONTAINED THEREIN. 